1. Field of the Invention
The present invention relates to batteries and more particularly to rechargeable, or secondary, batteries.
2. State of the Art
Lithium solid-polymer batteries of a type manufactured by the assignee are composed of a lithium metal anode, a single-phase, flexible solid polymer electrolyte and a cathode that stores lithium ions. The electrolyte is a solid, which reduces weight and volume and increases safety. The solid electrolyte battery operates efficiently over a broad temperature range, including room temperature.
In manufacture, cathode and electrolyte materials are coated onto a current collector to form a thin sheet called a laminate. A lithium foil anode is applied to the laminate to form a battery cell. These thin cells can be folded, rolled, or stacked, depending upon the application. A typical battery 10 of the type described is shown in FIG. 1, in which the current collector 21, the cathode 23, the polymer electrolyte 25 and the lithium anode 27 are shown in exploded view. Each solid-polymer battery cell is a bi-fold cell in which a continuous cell laminate 29 formed of the current collector and the cathode is folded about a piece of lithium metal foil 27 to form a "bi-fold cell," i.e., a dual cell formed by a folding process. The bi-fold cells 31a through 31n are stacked and connected in parallel to form a battery of the desired capacity. The current collectors of the cells are each connected to an electrode 33 to form a positive terminal of the battery, and the lithium anodes of each of the cells are all connected to an electrode 35 to form a negative terminal of the battery. Typically, the current collectors are made of aluminum and are spot-welded to a nickel electrode. A copper foil (not shown) is sandwiched between two lithium foils to form the lithium anodes, and the copper foils are spot-welded to another nickel electrode. The cell stack is then sealed in an aluminized outer wrap 37. Further details concerning manufacture of the battery may be found in U.S. patent application Ser. No. 08/073,195, filed Jun. 8, 1993, now U.S. Pat. No. 5,330,856, incorporated herein by reference.
The lithium anode, on discharge, supplies a flow of electrons to the external device. This current powers the external device and then returns to the cathode. Within the battery, the lithium anode and the cathode are separated by the solid polymer electrolyte. The solid polymer electrolyte acts as an electrical insulator, preventing electrons from moving between the lithium anode and the cathode inside the battery. However, the solid polymer electrolyte allows the movement of ions. As electrons are released from the lithium anode to the device, ions are released from the lithium anode, cross through the :solid polymer electrolyte and are stored in the cathode. During the recharge process, ions are transferred through the solid polymer electrolyte back to the lithium anode.
During recharging, the cells exhibit voltage plateaus as the cathode passes through different states as shown in FIG. 2. Ideally, all of the cells would exhibit identical charging characteristics. Because of process variations in both manufacture and chemistry, however, the individual cells will exhibit different efficiencies such that the voltage plateaus of the individual cells will not coincide. The cells therefore exhibit different back-EMFs and charge at different rates. If at some time one cell is in a state A shown in FIG. 2 so as to exhibit a low back-EMF at the same time as some or all of the other cells are in state B so as to exhibit relatively high back-EMFs, the charging rate of the one cell may exceed a recommended charging rate, thereby decreasing cell and battery life. In the extreme case, a sufficiently high charging rate may occur so as to pose a safety hazard. Similarly, under abusive conditions, such as a short-circuit condition, the consequent high current flow results in excess heat generation inside the cell. The heat can melt the lithium and vaporize the electrolyte, possibly resulting in the cell catching fire and burning.
What is needed, then, is a way of equalizing charge rates of individual battery cells, thereby prolonging battery life, and of increasing the safety of batteries of the type described.